Hygienic absorbent products such as disposable diapers contain, in addition to a water-permeable coverstock, a water-impermeable backsheet and one or more layers for distribution of liquid, an absorbent core typically comprising natural fibres such as cellulose fluff pulp fibres, synthetic fibres based on e.g. polyolefin and/or polyester and a superabsorbent polymer (SAP) material. In absorbent cores of this type, the synthetic fibres, which often are bicomponent fibres of e.g. polypropylene/polyethylene or polyester/polyethylene, are thermobonded to each other to form a supporting network for the core. Ideally, the synthetic fibres should be able to not only bond to each other, but also to the natural fibres and the SAP, so as to result in a core structure which is as strong and coherent as possible, and in which the natural fibres and the SAP are locked into place within the structure.
However, the existing synthetic fibres that are used for the production of drylaid, e.g. airlaid, nonwovens suffer from the disadvantage of suboptimal bonding to e.g. cellulose fibres. The problem is made worse by the fact that the natural fibres are typically relatively short, e.g. fluff pulp fibres with a length of not more than about 3 mm, as compared to the synthetic fibres, which are normally (although not necessarily) considerably longer. As a result, dust problems are created in the manufacturing process, and the performance of the resulting nonwovens is also suboptimal, since a large proportion of the natural fibres is not bonded to any of the synthetic fibres or otherwise held in place by means of the structure formed by bonding of the synthetic fibres.
It is therefore an object of the present invention to provide a bicomponent synthetic fibre which has an improved bonding affinity for natural fibres such as cellulose fluff pulp fibres and which therefore is particularly suitable for the production of drylaid nonwovens comprising a mixture of synthetic fibres and natural fibres.
EP 0465203-B1 discloses thermally bonded fibrous wet laid webs containing bicomponent fibres comprising a first component of polyester, polyamide or polypropylene and a second component of linear low density polyethylene (LLDPE) with a density of 0.88-0.945 g/cc and a grafted high density polyethylene (HDPE) with a density of 0.94-0.965 g/cc which has been grafted with maleic acid or maleic anhydride to provide succinic acid or succinic anhydride groups along the HDPE polymer.
EP 0421734-B1 discloses thermobondable bicomponent fibres composed of two different polyolefins having melting points which differ by at least 20.degree. C., the lower melting polyolefin containing 3-10% by weight of a monoglyceride of a fatty acid of 12 or more carbon atoms incorporated therein. The fibres are reported to be easily processable without the need for an oiling agent to be applied during spinning or drawing.
U.S. Pat. No. 4,950,541 discloses succinic acid and succinic anhydride grafts of linear ethylene polymers obtained by grafting maleic acid or maleic anhydride onto a LDPE (low density polyethylene), LLDPE or HDPE polymer. The grafted polymers are dyeable and can be used e.g. as the sheath component of a bicomponent fibre.
U.S. Pat. No. 4,684,576 discloses the production of blends of grafted HDPE with ungrafted LLDPE or LDPE, the HDPE having been grafted with maleic acid or maleic anhydride to provide succinic acid or succinic anhydride groups along the HDPE polymer. The blends are disclosed for use in producing laminate structures.
It has now unexpectedly been found that polyolefin bicomponent fibres whose low melting component comprises a non-grafted polyolefin component and a grafted polyolefin component which has been grafted with an unsaturated dicarboxylic acid or an anhydride thereof have advantageous properties when used in the production of drylaid nonwoven materials, including improved bonding to cellulose pulp fibres and improved strength properties in the resulting nonwovens.